This invention relates to highly grafted rubbers and rubber modified polymers produced therefrom.
Rubber modified polymers such as high impact polystyrene (HIPS) and acrylonitrile/butadiene/styrene (ABS) are typically produced by polymerizing styrene or styrene/acrylonitrile in the presence of a dissolved rubber, such that the rubber is dispersed within the polymer matrix in the form of discrete rubber particles containing occluded matrix polymer therein. The occluded rubber particles can have a variety of morphologies including lamellar (onion skin), cellular (multiple occlusions), and core shell (single occlusion). Rubber particles having small size cellular and core shell morphology are particularly advantageous in enhancing the balance of gloss and impact strength properties of the rubber modified polymer. Additionally, the physical properties of the rubber modified polymers can be enhanced with increased levels of grafting on the rubber.
Block copolymer rubbers can be highly grafted and have been used in previous rubber modified polymers. However, block copolymer rubbers are more expensive than non-block versions making this method economically unattractive.
There have been many attempts to increase the level of grafting on non-block rubbers. One approach includes hydroperoxidation of a rubber using singlet oxygen (SO) which results in an added number of reactive grafting sites on the rubber backbone. Generation of SO photochemically in a rubber/styrene mixture containing dissolved oxygen has been reported in U.S. Pat. No. 4,717,741 by Hahnfeld et al. However, the number of reactive grafting sites obtained is limited by the solubility of oxygen in the rubber/styrene mixture. Additionally, photosensitizers must be used which act as contaminants in the final polymer, causing discoloration. Additionally, solubilizers are used for the photosensitizer which end up in the recycle stream and must be separated from the styrene monomer, making this process economically unattractive.
Another approach involves generating SO in a rubber/styrene mixture by heating a compound which releases SO, such as bisquinone peroxide as described in U.S. Pat. No. 4,895,907 by Priddy et al. However, relatively low levels of grafting were achieved and bisquinone peroxides are not available for commercial use.
Yet another approach involves generating singlet oxygen from triphenylphosphite ozonides to increase the grafting onto a polybutadiene rubber for the preparation of polybutadiene-modified polystyrene, as disclosed in "Polybutadiene Hydroperoxide by Singlet Oxygen: Its Grafting and Morphology in Polystyrene Matrix", Journal of Applied Polymer Science, Vol. 31, 1827-1842 (1986) by Peng. However, the process described does not achieve high levels of grafting as hypothesized by Peng.
Therefore, it remains highly desirable to obtain rubber modified vinyl aromatic polymers containing highly grafted non-block rubber particles using an efficient and commercially viable process.